The following description is provided to assist the understanding of the reader. None of the information provided or references cited is admitted to be prior art to the present invention.
Starch enzymes such as amylases are currently used to make starch hydrolysates. Starch hydrolysates are used in the production of maltodextrins, corn syrups and sugars, and to make low viscosity starch pastes. The primary application for these pastes is in papermaking for size press or coating. High solids starch pastes are typically prepared either by chemical thinning or enzymatic conversion, which lower paste viscosity allowing higher solids to be used. In the latter use, starch hydrolytic enzymes, such as amylases, are added at doses and conditions sufficient to reduce the molecular weight of the cooked starch, but to maintain the composition primarily as a starch paste rather than a hydrolysate of mostly sugars. As a result, these pastes have high molecular weight components, which provide strength and film properties to the starch paste, but they also have a lower viscosity than unmodified starch pastes, meaning that pastes of higher solids can be made flowable or pumpable under typical operating conditions.
Under some conditions, starch pastes, especially acid thinned starch pastes, exhibit retrogradation and setback. Setback refers to the viscosity increase of a paste as temperature is lowered. Retrogradation refers to the irreversible insolubilization of starch paste with formation of a precipitate or gel depending on the concentration. Retrogradation is mainly due to the presence of amylose in the starch. The linear amylose molecules are attracted to each other and form bundles of parallel polysaccharide chains by the formation of hydrogen bonds between hydroxyl groups on neighboring molecules. The tendency of amylose molecules to retrograde is increased by molecular weight reduction to a chain length of around 100 glucose units. Amylopectin having a branched structure shows less tendency to retrograde.
The setback and retrogradation properties of pastes are affected by the levels and lengths of amylose (linear) and amylopectin (branched) fractions in the starch pastes. Amylose in native corn starch comprises about 27% of the starch and has a molecular weight average of about 2 million atomic weight units (AWU). Amylopectin comprises about 73% of corn starch and has an estimated molecular weight of 10 million. Different sources of starch will have different sizes and distributions of amylose and amylopectin. Likewise, thinning reactions will change the sizes and proportion of these components. In addition, derivatization reactions can also add branches to these molecules, which can change their behavior, specifically relative to setback and retrogradation.